1. Field of Invention
The invention relates to a hybrid vehicle and a control method thereof.
2. Description of Related Art
A conventional hybrid vehicle described in Japanese Patent Application Laid-Open No. HEI 11-315735 has a planetary gear unit that includes a sun gear, a ring gear and a carrier. The carrier is connected to an engine. The ring gear is connected to a drive wheel. The sun gear is connected to a generator motor. Rotation outputted from the ring gear and a drive motor is transferred to the drive wheel so as to produce drive force.
In this type of hybrid vehicle, in a low-rotation speed region, the drive motor torque, is greater than the engine torque. Therefore, at the time of starting the vehicle, only the drive motor is driven and the driving of the engine is stopped, so that the hybrid vehicle is run in a motor drive mode. During this operation, the generator motor is passively turned. When after the vehicle starts, the vehicle speed reaches an engine start vehicle speed Ve that is suitable for starting the engine, the engine is started by driving the generator motor to raise the engine revolution speed, to a revolution speed suitable for ignition (hereinafter, referred to as xe2x80x9cignition revolution speedxe2x80x9d). After that, the drive motor and the engine are driven, and the hybrid vehicle is run in a motor-engine drive mode.
When a vehicle speed of the conventional hybrid vehicle is low, the generator motor is passively turned at a low speed, so that a generator motor torque that is sufficient to raise the engine revolution speed to the ignition revolution speed can be produced. However, when the vehicle speed is high, the generator motor is passively turned at a high speed, so that a sufficient generator motor torque cannot be produced due to the characteristic of the generator motor. As a result, the engine revolution speed cannot be raised to the ignition revolution speed. In that case, the engine cannot be started.
It is an object of the invention to solve the aforementioned problem of the conventional hybrid vehicle and to provide a hybrid vehicle and a control method thereof capable of reliably starting an engine.